6 Spatial Data and Maps
We’ll explore the basics of simple features (sf) for building spatial datasets, then some common mapping methods, probably:
- ggplot2
- tmap
6.1 Spatial Data
To work with spatial data requires extending R to deal with it using packages. Many have been developed, but the field is starting to mature using international open GIS standards.
sp (until recently, the dominant library of spatial tools)
- Includes functions for working with spatial data
- Includes
spplotto create maps - Also needs
rgdalpackage forreadOGR– reads spatial data frames.
sf (Simple Features)
- ISO 19125 standard for GIS geometries
- Also has functions for working with spatial data, but clearer to use.
- Doesn’t need many additional packages, though you may still need
rgdalinstalled for some tools you want to use. - Replacing
spandspplotthough you’ll still find them in code. We’ll give it a try… - Works with ggplot2 and tmap for nice looking maps.
Cheat sheet: https://github.com/rstudio/cheatsheets/raw/master/sf.pdf
6.1.0.1 simple feature geometry sfg and simple feature column sfc
6.1.1 Examples of simple geometry building in sf
sf functions have the pattern st_*
st means “space and time”
See Geocomputation with R at https://geocompr.robinlovelace.net/ or https://r-spatial.github.io/sf/ for more details, but here’s an example of manual feature creation of sf geometries (sfg):
library(tidyverse)
library(sf)
library(iGIScData)[As usual, go to the relevant project, in this case generic_methods]
library(sf)
eyes <- st_multipoint(rbind(c(1,5), c(3,5)))
nose <- st_point(c(2,4))
mouth <- st_linestring(rbind(c(1,3),c(3, 3)))
border <- st_polygon(list(rbind(c(0,5), c(1,2), c(2,1), c(3,2),
c(4,5), c(3,7), c(1,7), c(0,5))))
face <- st_sfc(eyes, nose, mouth, border) # sfc = sf column
plot(face)
Figure 6.1: Building simple geometries in sf
The face was a simple feature column (sfc) built from the list of sfgs. An sfc just has the one column, so is not quite like a shapefile.
- But it can have a coordinate referencing system CRS, and so can be mapped.
- Kind of like a shapefile with no other attributes than shape
[westUS]
6.1.2 Building a mappable sfc from scratch
CA_matrix <- rbind(c(-124,42),c(-120,42),c(-120,39),c(-114.5,35),
c(-114.1,34.3),c(-114.6,32.7),c(-117,32.5),c(-118.5,34),c(-120.5,34.5),
c(-122,36.5),c(-121.8,36.8),c(-122,37),c(-122.4,37.3),c(-122.5,37.8),
c(-123,38),c(-123.7,39),c(-124,40),c(-124.4,40.5),c(-124,41),c(-124,42))
NV_matrix <- rbind(c(-120,42),c(-114,42),c(-114,36),c(-114.5,36),
c(-114.5,35),c(-120,39),c(-120,42))
CA_list <- list(CA_matrix); NV_list <- list(NV_matrix)
CA_poly <- st_polygon(CA_list); NV_poly <- st_polygon(NV_list)
sfc_2states <- st_sfc(CA_poly,NV_poly,crs=4326) # crs=4326 specifies GCS
st_geometry_type(sfc_2states)## [1] POLYGON POLYGON
## 18 Levels: GEOMETRY POINT LINESTRING POLYGON MULTIPOINT MULTILINESTRING MULTIPOLYGON GEOMETRYCOLLECTION ... TRIANGLElibrary(tidyverse)
ggplot() + geom_sf(data = sfc_2states)
Figure 6.2: A simple map built from scratch with hard-coded data as simple feature columns
sf class
Is like a shapefile: has attributes to which geometry is added, and can be used like a data frame.
attributes <- bind_rows(c(abb="CA", area=423970, pop=39.56e6),
c(abb="NV", area=286382, pop=3.03e6))
twostates <- st_sf(attributes, geometry = sfc_2states)
ggplot(twostates) + geom_sf() + geom_sf_text(aes(label = abb))## Warning in st_point_on_surface.sfc(sf::st_zm(x)): st_point_on_surface may not give correct results for longitude/latitude data
Figure 6.3: Using an sf class to build a map, displaying an attribute
6.1.3 Creating features from shapefiles or tables
sf’s st_read reads shapefiles
- shapefile is an open GIS format for points, polylines, polygons
You would normally have shapefiles (and all the files that go with them – .shx, etc.)
stored on your computer, but we’ll access one from the iGIScData external data folder [sierra]:
library(iGIScData)
library(sf)
shpPath <- system.file("extdata","CA_counties.shp", package="iGIScData")
CA_counties <- st_read(shpPath)## Reading layer `CA_counties' from data source `C:\Users\900008452\Documents\R\win-library\4.0\iGIScData\extdata\CA_counties.shp' using driver `ESRI Shapefile'
## Simple feature collection with 58 features and 60 fields
## Geometry type: MULTIPOLYGON
## Dimension: XY
## Bounding box: xmin: -124.4152 ymin: 32.53427 xmax: -114.1312 ymax: 42.00952
## Geodetic CRS: WGS 84plot(CA_counties)## Warning: plotting the first 9 out of 60 attributes; use max.plot = 60 to plot all
st_as_sf converts data frames
- using coordinates read from x and y variables, with crs set to coordinate system (4326 for GCS)
sierraFebpts <- st_as_sf(sierraFeb, coords = c("LONGITUDE", "LATITUDE"), crs=4326)
plot(sierraFebpts)
[air_quality]
library(tidyverse)
library(sf)
library(iGIScData)
censusCentroids <- st_centroid(BayAreaTracts)
TRI_sp <- st_as_sf(TRI_2017_CA, coords = c("LONGITUDE", "LATITUDE"),
crs=4326) # simple way to specify coordinate reference
bnd <- st_bbox(censusCentroids)
ggplot() +
geom_sf(data = BayAreaCounties, aes(fill = NAME)) +
geom_sf(data = censusCentroids) +
geom_sf(data = CAfreeways, color = "grey") +
geom_sf(data = TRI_sp, color = "yellow") +
coord_sf(xlim = c(bnd[1], bnd[3]), ylim = c(bnd[2], bnd[4])) +
labs(title="Bay Area Counties, Freeways and Census Tract Centroids")
Figure 6.4: ggplot map of Bay Area TRI sites, census centroids, freeways
6.1.4 Coordinate Referencing System
Say you have data you need to make spatial with a spatial reference
sierra <- read_csv("sierraClimate.csv")
EPSG or CRS codes are an easy way to provide coordinate referencing.
Two ways of doing the same thing.
- Spell it out:
GCS <- "+proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0"
wsta = st_as_sf(sierra, coords = c("LONGITUDE","LATITUDE"), crs=GCS)- Google to find the code you need and assign it to the crs parameter:
wsta <- st_as_sf(sierra, coords = c("LONGITUDE","LATITUDE"), crs=4326)
6.1.4.1 Removing Geometry
There are many instances where you want to remove geometry from a sf data frame
Some R functions run into problems with geometry and produce confusing error messages, like “non-numeric argument”
You’re wanting to work with an sf data frame in a non-spatial way
One way to remove geometry:
myNonSFdf <- mySFdf %>% st_set_geometry(NULL)
6.1.5 Spatial join st_join
A spatial join with st_join
joins data from census where TRI points occur [air_quality]
TRI_sp <- st_as_sf(TRI_2017_CA, coords = c("LONGITUDE", "LATITUDE"), crs=4326) %>%
st_join(BayAreaTracts) %>%
filter(CNTY_FIPS %in% c("013", "095"))6.1.6 Plotting maps in the base plot system
There are various programs for creating maps from spatial data, and we’ll look at a few after we’ve looked at rasters. As usual, the base plot system often does something useful when you give it data.
plot(BayAreaCounties)## Warning: plotting the first 9 out of 60 attributes; use max.plot = 60 to plot all
And with just one variable:
plot(BayAreaCounties["POP_SQMI"])
There’s a lot more we could do with the base plot system, but we’ll mostly focus on some better options in ggplot2 and tmap.
6.2 Raster GIS in R
Simple Features are feature-based, so based on the name I guess it’s not surprising that sf doesn’t have support for rasters. But we can use the raster package for that.
A bit of raster reading and map algebra with Marble Mountains elevation data [marbles]
library(raster)## Warning: package 'raster' was built under R version 4.0.4## Warning: package 'sp' was built under R version 4.0.4rasPath <- system.file("extdata","elev.tif", package="iGIScData")
elev <- raster(rasPath)
slope <- terrain(elev, opt="slope")
aspect <- terrain(elev, opt="aspect")
slopeclasses <-matrix(c(0,0.2,1, 0.2,0.4,2, 0.4,0.6,3,
0.6,0.8,4, 0.8,1,5), ncol=3, byrow=TRUE)
slopeclass <- reclassify(slope, rcl = slopeclasses)
plot(elev)
plot(slope)
plot(slopeclass)
plot(aspect)
6.2.1 Raster from scratch
new_raster2 <- raster(nrows = 6, ncols = 6, res = 0.5,
xmn = -1.5, xmx = 1.5, ymn = -1.5, ymx = 1.5,
vals = 1:36)
plot(new_raster2)
6.3 ggplot2 for maps
The Grammar of Graphics is the gg of ggplot.
- Key concept is separating aesthetics from data
- Aesthetics can come from variables (using aes()setting) or be constant for the graph
Mapping tools that follow this lead
- ggplot, as we have seen, and it continues to be enhanced
- tmap (Thematic Maps) https://github.com/mtennekes/tmap Tennekes, M., 2018, tmap: Thematic Maps in R, Journal of Statistical Software 84(6), 1-39
ggplot(CA_counties) + geom_sf()
Try ?geom_sf and you’ll find that its first parameters is mapping with aes() by default. The data property is inherited from the ggplot call, but commonly you’ll want to specify data=something in your geom_sf call.
Another simple ggplot, with labels
ggplot(CA_counties) + geom_sf() +
geom_sf_text(aes(label = NAME), size = 1.5)## Warning in st_point_on_surface.sfc(sf::st_zm(x)): st_point_on_surface may not give correct results for longitude/latitude data
and now with fill color
ggplot(CA_counties) + geom_sf(aes(fill = MED_AGE)) +
geom_sf_text(aes(label = NAME), col="white", size=1.5)## Warning in st_point_on_surface.sfc(sf::st_zm(x)): st_point_on_surface may not give correct results for longitude/latitude data
Repositioned legend, no “x” or “y” labels
ggplot(CA_counties) + geom_sf(aes(fill=MED_AGE)) +
geom_sf_text(aes(label = NAME), col="white", size=1.5) +
theme(legend.position = c(0.8, 0.8)) +
labs(x="",y="")
Map in ggplot2, zoomed into two counties [air_quality]:
library(tidyverse); library(sf); library(iGIScData)
census <- BayAreaTracts %>%
filter(CNTY_FIPS %in% c("013", "095"))
TRI <- TRI_2017_CA %>%
st_as_sf(coords = c("LONGITUDE", "LATITUDE"), crs=4326) %>%
st_join(census) %>%
filter(CNTY_FIPS %in% c("013", "095"),
(`5.1_FUGITIVE_AIR` + `5.2_STACK_AIR`) > 0)## although coordinates are longitude/latitude, st_intersects assumes that they are planar
## although coordinates are longitude/latitude, st_intersects assumes that they are planarbnd = st_bbox(census)
ggplot() +
geom_sf(data = BayAreaCounties, aes(fill = NAME)) +
geom_sf(data = census, color="grey40", fill = NA) +
geom_sf(data = TRI) +
coord_sf(xlim = c(bnd[1], bnd[3]), ylim = c(bnd[2], bnd[4])) +
labs(title="Census Tracts and TRI air-release sites") +
theme(legend.position = "none")
6.3.1 Rasters in ggplot2
Raster display in ggplot2 is currently a little awkward, as are rasters in general in the feature-dominated GIS world.
We can use a trick: converting rasters to a grid of points [marbles:
library(tidyverse)
library(sf)
library(raster)
rasPath <- system.file("extdata","elev.tif", package="iGIScData")
elev <- raster(rasPath)
shpPath <- system.file("extdata","trails.shp", package="iGIScData")
trails <- st_read(shpPath)## Reading layer `trails' from data source `C:\Users\900008452\Documents\R\win-library\4.0\iGIScData\extdata\trails.shp' using driver `ESRI Shapefile'
## Simple feature collection with 32 features and 8 fields
## Geometry type: LINESTRING
## Dimension: XY
## Bounding box: xmin: 481903.8 ymin: 4599196 xmax: 486901.9 ymax: 4603200
## Projected CRS: NAD83 / UTM zone 10Nelevpts = as.data.frame(rasterToPoints(elev))
ggplot() +
geom_raster(data = elevpts, aes(x = x, y = y, fill = elev)) +
geom_sf(data = trails)
6.4 tmap
Basic building block is tm_shape(data) followed by various layer elements such as tm_fill() shape can be features or raster See Geocomputation with R Chapter 8 “Making Maps with R” for more information. https://geocompr.robinlovelace.net/adv-map.html
library(spData)## Warning: package 'spData' was built under R version 4.0.4## To access larger datasets in this package, install the spDataLarge package with: `install.packages('spDataLarge',
## repos='https://nowosad.github.io/drat/', type='source')`##
## Attaching package: 'spData'## The following object is masked _by_ '.GlobalEnv':
##
## elevlibrary(tmap)## Warning: package 'tmap' was built under R version 4.0.4tm_shape(world) + tm_fill() + tm_borders()
Color by variable [air_quality]
library(sf)
library(tmap)
tm_shape(BayAreaTracts) + tm_fill(col = "MED_AGE")
tmap of sierraFeb with hillshade and point symbols [sierra]
library(tmap)
library(sf)
library(raster)
library(iGIScData)
tmap_mode("plot")## tmap mode set to plottingtmap_options(max.categories = 8)
sierra <- st_as_sf(sierraFeb, coords = c("LONGITUDE", "LATITUDE"), crs = 4326)
rasPath <- system.file("extdata","ca_hillsh_WGS84.tif", package="iGIScData")
hillsh <- raster(rasPath)
bounds <- st_bbox(sierra)
tm_shape(hillsh,bbox=bounds)+
tm_raster(palette="-Greys",legend.show=FALSE,n=10) + tm_shape(sierra) + tm_symbols(col="TEMPERATURE",
palette=c("blue","red"), style="cont",n=8) +
tm_legend() +
tm_layout(legend.position=c("RIGHT","TOP"))## stars object downsampled to 1092 by 915 cells. See tm_shape manual (argument raster.downsample)
Note: “-Greys” needed to avoid negative image, since “Greys” go from light to dark, and to match reflectance as with b&w photography, they need to go from dark to light.
6.4.1 Interactive Maps
The word “static” in “static maps” isn’t something you would have heard in a cartography class 30 years ago, since essentially all maps then were static. Very important in designing maps is considering your audience, and one characteristic of the audience of those maps of yore were that they were printed and thus fixed on paper. A lot of cartographic design relates to that property:
- Figure-to-ground relationships assume “ground” is a white piece of paper (or possibly a standard white background in a pdf), so good cartographic color schemes tend to range from light for low values to dark for high values.
- Scale is fixed and there are no “tools” for changing scale, so a lot of attention must be paid to providing scale information.
- Similarly, without the ability to see the map at different scales, inset maps are often needed to provide context.
Interactive maps change the game in having tools for changing scale, and always being “printed” on a computer or device where the color of the background isn’t necessarily white. We are increasingly used to using interactive maps on our phones or other devices, and often get frustrated not being able to zoom into a static map.
A widely used interactive mapping system is Leaflet, but we’re going to use tmap to access Leaflet behind the scenes and allow us to create maps with one set of commands. The key parameter needed is tmap_mode which must be set to “view” to create an interactive map. [air_quality]
tmap_mode("view")## tmap mode set to interactive viewingtm_shape(BayAreaTracts) + tm_fill(col = "MED_AGE", alpha = 0.5)